Absorbent articles comprising acidic cellulosic fibers and an organic zinc salt

ABSTRACT

An absorbent article, such as a diaper, panty diaper, sanitary napkin or incontinence device includes a liquid-permeable topsheet, a backsheet and an absorbent core enclosed between the liquid-permeable topsheet and the backsheet. The absorbent core includes acidic fluff pulp having a pH of 5.5 or less and an organic zinc salt, in particular zinc ricinoleate. The combination of organic zinc salt and acidic fluff pulp exerts a synergetic effect in the suppression of ammonia.

The present invention relates to an absorbent article such as a diaper,panty diaper, sanitary napkin or incontinence device, which comprises aneffective odour control system, and odour-controlling cellulosic fiberswhich can be used in such absorbent articles. The present inventionrelates in particular to such absorbent articles wherein acidiccellulosic fibers, such as acidic fluff pulp having a pH 5.5 or less,and an organic zinc salt such a zinc ricinoleate interact favorably, inparticular synergetically to reduce malodours such as ammonia.

TECHNICAL BACKGROUND

One important area of development in the area of absorbent articles ofthe above-mentioned type is the control of odourous compounds formingtypically after the release of body fluids, especially over a longerperiod of time. These compounds include fatty acids, ammonia, amines,sulphur-containing compounds and ketones and aldehydes. They are presentas natural ingredients of body fluids or result from degradationprocesses of natural ingredients such as urea, which is broken down bymicroorganisms or bacteria occurring in the urogenital flora to ammonia.

Various approaches exist to suppress the formation of unpleasant odoursin absorbent articles. WO 97/46188, WO 97/46190, WO 97/46192, WO97/46193, WO 97/46195 and WO 97/46196 teach for instance theincorporation of odour inhibiting additives or deodorants such aszeolites and silica. The absorption of bodily liquids reduces howeverthe odour inhibiting capacity of zeolites as soon as these becomesaturated with water, as mentioned for instance in WO 98/17239.

A second approach involves the addition of lactobacilli with theintention of inhibiting malodour-forming bacteria in the product. Theincorporation of lactobacilli and their favourable effect are disclosedfor instance in SE 9703669-3, SE 9502588-8, WO 92/13577, SE 9801951-6and SE 9804390-4.

Moreover, it is known from WO 98/57677, WO 00/35503 and WO 00/35505 thatpartially neutralized superabsorbent materials (acidic superabsorbentmaterials) counteract the formation of unpleasant odours in absorbentarticles. However, acidic superabsorbent materials absorb lower amountsof body fluid compared to regular superabsorbent materials (in thefollowing also referred to as superabsorbent polymer, SAP). Theabsorbent articles described in the above-mentioned WO 98/57677 mayadditionally contain fluffed cellulose pulp having a pH value below 7,preferably below 6.

Further, U.S. Pat. No. 6,852,904 describes cellulose fibers treated withacidic odor control agents and their use in absorbent products.

Various known odour control systems are however not effective enough orloose their effectiveness too quickly to be accepted by consumers ofabsorbent products.

Therefore, an ongoing demand exists in the art for effectiveodour-control systems in absorbent articles.

From other technical areas it is further known that organic zinc saltsof unsaturated hydroxylated fatty acids such as zinc ricinoleate aredeodorizing active ingredients (see for instance DE 1792074 A1, DE2548344 A1 and DE 3808114 A1).

It is one technical object of the present invention to overcomedeficiencies discussed above in connection with the prior art.

It is one further technical object to provide an absorbent articlehaving an efficient odour control system.

It is one further technical object of the present invention toconsiderably reduce or eliminate ammonia formation in absorbentarticles.

Further objects will become apparent from the following description ofthe invention.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to an absorbent article, such as a diaper,panty diaper, panty liner, sanitary napkin or incontinence devicecomprising a liquid-permeable topsheet, a (preferablyliquid-impermeable) backsheet and an absorbent core enclosed betweensaid liquid-permeable topsheet and said backsheet, wherein saidabsorbent core comprises acidic cellulosic fibers, in particular acidicfluff pulp fibers, having a pH value of 5.5 or less, and an organic zincsalt, in particular the zinc salt of a monocarboxylic acid.

The present invention also relates to acidic cellulosic fibers having apH of 5.5 or less characterized in that they comprise the zinc salt of amonocarboxylic acid and their use for odour control, especially in theaforementioned absorbent articles.

In the present specification, the acidic cellulosic fibers (CF) having apH value of 5.5 or less are oftentimes simply referred to as “acidiccellulosic fibers (CF)” and acidic fluff pulp having a pH value of 5.5or less as “acidic fluff pulp”.

The present inventors have found that the acidic CF, in particular fluffpulp, and organic zinc salt, in particular the zinc salt of amonocarboxylic acid such as zinc ricinoleate, interact in thesuppression of ammonia while preferably keeping the natural bacterialflora in the urogenital region, and they completed the present inventionbased on this finding.

Without wishing to be bound by theory, the mechanism underlying theodour reduction of the present invention is assumed to be as follows. Itwas found that the ammonia which produces the malodour in absorbentproducts, such as incontinence products is formed in the following way:

Bacteria+Urea→NH₃

In the present invention, the acidic CF, in particular fluff pulp fibershave the function of making the environment unfavourable for thebacteria while the organic zinc salt, e.g. the zinc ricinoleate removesthe ammonia (NH₃) actually formed.

The aim of the present invention is to develop an absorbent articlewhere the amount of unwanted bacteria or microorganisms, such asammonia-producing bacteria does not increase during use.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the specification and claims, the use of “comprising”isintended to cover also the more restricting meanings “essentiallyconsisting of” and “consisting of”.

As “absorbent article” we understand articles capable of absorbing bodyfluids such as urine, watery feces, female secretion or menstrualfluids. These absorbent articles include, but are not limited todiapers, panty diapers, panty liners, sanitary napkins or incontinencedevice (as used for instance for adults).

Such absorbent articles have a liquid-pervious topsheet, which duringuse is facing the wearer's body. They further comprise a (preferablyliquid-impervious) backsheet, for instance a plastic film, aplastic-coated nonwoven or a hydrophobic nonwoven and an absorbent coreenclosed between the liquid-pervious topsheet and the backsheet.

A suitable topsheet may be manufactured from a wide range of materialssuch as woven and nonwoven materials (e.g. a nonwoven web of fibers),polymeric materials such as apertured plastic films, e.g. aperturedformed thermoplastic films and hydroformed thermoplastic films; porousfoams; reticulated foams; reticulated thermoplastic films; andthermoplastic scrims. Suitable woven and nonwoven materials can becomprised of natural fibers (e.g. wood or cotton fibers), syntheticfibers (e.g. polymeric fibers such as polyesters, polypropylene orpolyethylene fibers) or from a combination of natural and syntheticfibers. When the topsheet comprises a nonwoven web, the web may bemanufactured by a wide number of known techniques. For example, the webmay be spun-bonded, carded, wet-laid, melt-blown, hydroentangled,combinations of the above or the like. In accordance with the invention,it is preferred to make use of apertured plastic films (e.g.thermoplastic films) or nonwoven materials based on synthetic fibers,e.g. those made from polyethylene or polypropylene homo- or copolymersand polymer compositions based thereon.

Optionally, at least one further layer exists between the absorbent coreand the topsheet and may be made from hydrophobic and hydrophilic web orfoam materials. As “web material” we understand coherent flatfiber-based structures of paper tissue, woven or nonwoven type. Thenonwoven material may have the same features as described above fortopsheets.

Specifically, the at least one further layer may contribute to fluidmanagement, for instance in the form of at least oneacquisition/distribution layer. Such structures are taught for instanceby U.S. Pat. No. 5,558,655, EP 0 640 330 A1, EP 0 631 768 A1 or WO95/01147.

“Foam materials” are also well known in the art and for instancedescribed in EP 0 878 481 A1 or EP 1 217 978 A1 in the name of thepresent applicant.

The absorbent core, which may be partially or totally surrounded by acore wrap, comprises acidic cellulosic fibers, in particular acidicfluff pulp fibers, having a pH value of 5.5 or less.

The term “cellulosic fibers” also referred to as “CF” relates to fibersfrom wood, woody plants and certain non-woody plants and cellulose-basedrecycled and regenerated fibers. Woody plants include for instancedeciduous (hardwood) and coniferous (softwood) trees. Non-woody plantsinclude for instance cotton, flax, esparto, grass, milkweed, straw, jutehemp and bagasse. The cellulosic fibers are preferably “pulp fibers”.

The term “pulp fibers” includes chemical pulp and mechanical pulpfibers.

According to DIN 6730, “chemical pulp” is a fibrous material obtainedfrom plant raw materials from which most non-cellulose components havebeen removed by chemical pulping without substantial mechanicalpost-treatment. In case of chemical pulping processes such as thesulfite or sulfate (Kraft) process, primarily the lignin components andthe hemi-cellulose components are dissolved from the wood to varyingdegrees depending on the field of application of the chemical pulp. Theresult is a fibrous material consisting primarily of cellulose.

“Mechanical pulp” is the general term for fibrous materials made of woodentirely or almost entirely by mechanical means, optionally at increasedtemperatures. Mechanical pulp is subdivided into the purely mechanicalpulps (groundwood pulp and refiner mechanical pulp) as well asmechanical pulps subjected to chemical pretreatment: chemo-mechanicalpulp (CMP), such as chemo-thermomechanical pulp (CTMP).

The starting pulps which may be used in the present invention may relateto primary fibrous materials (raw pulps) or to secondary fibrousmaterials, whereby a secondary fibrous material is defined as a fibrousraw material recovered from a recycling process. The primary fibrousmaterials may relate both to a chemically digested pulp and tomechanical pulp such as thermorefiner mechanical pulp (TMP),chemothermorefiner mechanical pulp (CTMP) or high temperaturechemithermomechanical pulp (HTCTMP). Synthetic cellulose-containingfibers can also be used. Preference is nevertheless given to the use ofpulp from plant material, particularly wood-forming plants. Fibers ofsoftwood (usually originating from conifers), hardwood (usuallyoriginating from deciduous trees) or from cotton linters can be used forexample. Fibers from esparto (alfa) grass, bagasse (cereal straw, ricestraw, bamboo, hemp), kemp fibers, flax and other woody and cellulosicfiber sources can also be used as raw materials. The corresponding fibersource is chosen in accordance with the desired properties of theabsorbent core, such as softness and absorption capacity in a mannerknown in the art. With regard to the softness of the products, the useof chemical raw pulps is also preferred, whereby it is possible to usecompletely bleached, partially bleached and unbleached fibers. Thechemical raw pulps suitable according to the invention include, interalia, sulfite pulps, kraft pulps (sulfate process), soda pulps (cookingwith sodium hydroxide), pulps from high-pressure cooking with organicsolvents (e.g. Organosolv, Organocell, Acetosolv, Alcell) and pulps frommodified processes (e.g. ASAM, Stora or Sivola process). Among the kraftpulps, it is possible to use those which were obtained in continuouscooking systems (MCC (modified continuous cooking), EMCC (extendedmodified continuous cooking) and ITC (isothermal cooking)). The productsof discontinuous kraft processes (e.g. RDH (rapid displacement heating),Superbatch and Enerbatch) are also suitable as a starting product. Thesulfite processes include the acidic sulfite/bisulfite processes,bisulfite process, “neutral sulfite semi-chemical pulping” (NSSC)process and alkaline sulfite processes such as processes in which inaddition to aqueous alkali, sulfite and/or anthraquinone in combinationwith organic solvents such as methanol were used for cooking, e.g. theso-called ASAM process (alkali sulfite anthraquinone methanol). Themajor difference between the acidic and neutral or alkaline sulfiteprocesses is the higher degree of delignification in acidic cookingprocesses (lower kappa numbers). The NSSC process provides semi-chemicalpulps which are advantageously defibered in downstream mechanicalfibrillation before they are used according to the invention for thepurpose of oxidation. The sulfite and kraft pulps considerably differ interms of their fibrous material properties. The individual fiberstrengths of sulfite pulps are usually much lower than those of kraftpulps. The mean pore width of the swollen fibers is also greater insulfite pulps and the density of the cell wall is lower compared tosulfate pulps, which simultaneously means that the cell-wall volume isgreater in sulfite pulps. For this reason, there are also obviousdifferences regarding water absorption and swelling behavior of thecellulosic fibrous materials, which must also be taken intoconsideration when selecting a material for the absorbent core.

For the purpose of the present invention, general cellulosic fibers, inparticular pulp fibers as described above are also referred to as“standard CF” or “non-acidic CF”.

The cellulosic fibers to be used in the absorbent core are preferablyfluff pulp fibers. The term “fluff pulp fibers” as used herein is wellknown in the art of making paper and absorbent products. It refers to avariant of “standard CF” as described above which is characterized byits fluffy state which can be achieved by comminuting standard, chemical(e.g. Kraft or sulfite), mechanical (e.g. groundwood pulp and refinermechanical pulp) or chemomechanical pulp (CMP), such as TMP, CTMP orHTCTMP. Preferably chemical or chemomechanical pulp, optionally in ableached form is used for the preparation of fluff pulp. Fluff pulp maycomprise mainly, preferably exclusively, softwood fibers which impartthe necessary softness for use in absorbent products. Suitable wood pulpfibers for manufacturing fluff pulp are e.g. Southern Softwood Kraft andNorthern Softwood Sulphite. There are various grades of fluff pulps,such as debonded, also called treated, fluff pulps which are softer thanregular fluff. Main producers of fluff pulp are Weyerhaeuser Co. andGeorgia Pacific Corp. in the U.S. and Finland-based Stora Enso Oy. Forthe purpose of the present invention, general fluff pulp as describedabove is also referred to as “standard fluff pulp” or “non-acidic fluffpulp”. In the following, “fluff pulp” and “fluff CF” will be used assynonyma.

The pH value of standard CF, including standard fluff pulp variessignificantly, e.g. depending on the production method. Generally,standard (fluff) CF have a pH of from above 5.5 to 6.5, preferablyaround 6. Unlike standard (fluff) CF, the acidic (fluff) CF for use inthe present invention have a pH of 5.5 or less. For removing bacteria, apH value of 5.0 or less is advantageous. The pH value of the acidic(fluff) CF is preferably 2.0 to 5.0, more preferably 2.5 to 4.5, stillmore preferably 3.0 to 4.0 and most preferably 3.2 to 3.6. The pH of CFcan be measured using the standard test Tappi T 509-02, in particularTappi method T 509 om-02.

The acidic (fluff) CF fibers may also be admixed with standard (fluff)CF and/or superabsorbent polymer material (SAP).

In the corresponding absorbent core and, if applicable, each layerthereof, the total amount of cellulosic fibers, i.e. acidic (fluff) CFor a mixture of acidic and non-acidic (fluff) CF, is preferably 90 to 30wt.-%, more preferably 80 to 35 wt.-%, in particular 70 to 40 wt.-%, forinstance 70 to 50 wt.-%, based on the weight of the entire mixture of(fluff) CF and superabsorbent materials (without organic zinc salt). Theterm “(fluff) CF” is used as abbreviation for “non-fluffed cellulosicfibers such as non-fluffed pulp and/or fluff cellulosic fibers, i.e.fluff pulp”

If used in admixture, the weight ratio of acidic (fluff) CF andnon-acidic (fluff) CF is not particularly restricted (e.g. 5/95 to 95/5,10/90 to 90/10, 20/80 to 80/20). Accordingly, weight ratios of acidic(fluff) CF/non-acidic (fluff) CF of 100/0 to 50/50 (e.g. 95/5 to 60/40,90/10 to 70/30) can be preferably selected depending on the propertiesto be achieved.

As indicated above, the absorbent core may further comprise asuperabsorbent material, which may be acidic or non-acidic. According toone embodiment, the absorbent core contains an acidic superabsorbentmaterial having a pH value of 5.5 or less (measured according to EDANAWSP 200.2), and according to a second alternative embodiment, theabsorbent core does not contain such a material.

The total amount of the superabsorbent material may be 10 to 70 weight%, based on the weight of the core (excluding the organic zinc salt).

The term “superabsorbent material” is well known in the art anddesignates water-swellable, water-insoluble materials capable ofabsorbing the multiple of their own weight in body fluids. Preferably,the superabsorbent material is capable of absorbing at least about 10times its weight, preferably at least about 15 times its weight, inparticular at least about 20 times its weight in an aqueous solutioncontaining 0.9 wt.-% of sodium chloride (under usual measuringconditions where the superabsorbent surface is freely accessible to theliquid to be absorbed). To determine the absorption capacity of thesuperabsorbent material, the standard test EDANA WSP 241.2 can be used.

The acidic and non-acidic superabsorbent materials can be distinguishedby way of their pH value. While non-acidic SAPs (also referred to asstandard SAPs) have a pH which lies e.g. in a range of 5.8 or more,acidic SAPs have a pH of 5.5 or less. Consequently, non-acidic SAPs mayincrease the pH in the absorbent core comprising acidic (fluff) CFaccording to the invention. Therefore, when acidic (fluff) CF accordingto the invention are used along with non-acidic SAPs, the pH of theacidic (fluff) CF used is preferably low enough to achieve a pH of theabsorbent core of 5.5 or less, preferably 5.0 or less, more preferablyfrom 3.0 to 5.0 after wetting. The pH of the absorbent core is measuredaccording to the test method A described in the examples. While the pHof the acidic (fluff) CF to achieve the above pH of the absorbent coredepends on the relative amount of acidic fluff pulp and non-acidic SAPin the absorbent core, the acidic (fluff) CF preferably have a pH valueof 2.5 to 4.5, preferably from 3.0 to 4.0 and most preferably from 3.2to 3.6 in this case.

In addition to the above materials, i.e. acidic (fluff) CF andoptionally non-acidic (fluff) CF and superabsorbent material, theabsorbent core may comprise, in admixture, other absorbent materials.Any other absorbent material that is generally compressible,conformable, non-irritating to the wearer's skin and capable ofabsorbing and retaining liquids such as urine and other body exudatescan be used. Examples of other absorbent materials to be incorporated inthe absorbent core include a wide variety of liquid-absorbent materialscommonly used in disposable diapers and other absorbent articles such ascreped cellulose wadding; melt blown polymers, including co-form;chemically stiffened, modified or cross-linked cellulosic fibers;tissue, including tissue wraps and tissue laminates, absorbent foams,absorbent sponges, absorbent gelling materials, or any other knownabsorbent materials or combinations of materials.

As indicated before, the absorbent core in the absorbent article of theinvention may also contain fibers others than acidic CF, such as acidicfluff pulp fibers. These other fibers are preferably also capable ofabsorbing body liquid as is the case for hydrophilic fibers. Mostpreferably the fibers are other cellulosic fibers such as standard fluffpulp, cotton, cotton linters, rayon, cellulose acetate and the like. Thestandard fluff pulp can be of the above-described mechanical or chemicaltype, the chemical pulp being preferred.

There are no specific restrictions as to the method of producing theacidic (fluff) cellulosic fibers for use in the present invention.According to a preferred embodiment, the acidic (fluff) CF are obtainedby treating standard CF with an acidifying agent. If an acidifying agentis used the same differs structurally from the organic zinc salt.

The acidifying agents for use in the present invention are notspecifically limited in kind, as long as they do not disintegrate ordecompose the standard fluff pulp being treated. One example isSO₂-water. Preferably the acidifying agent is a suitable acid, e.g. aweak acid or a salt thereof. The use of halogen-free non-oxidizing acidsis preferred. Suitable acids are those which when incorporated in thestandard (fluff) CF will not release any substances which may be harmfulor acrid to skin. It should be noted that the skin in the region whichcomes into contact with absorbent articles is very sensitive, in infantsand adults, alike. Hence, the acid used as an acidifying agent ispreferably one that is approved of or admitted for use in food and/orcosmetics.

Preferably the acidifying agent is selected from optionallyhydroxyl-substituted mono- and polycarboxylic acids, their salts, andmixtures thereof. The mono- or polycarboxylic acid may be aliphatic oraromatic. The salt is preferably an alkali metal (e.g. K or Na) or earthalkaline metal salt (e.g. Ca or Mg). If used in salt form, theacidifying agent, preferably the optionally hydroxyl-substituted mono-and polycarboxylic acid is only partially neutralized to provide acidicsolutions in water.

The optionally hydroxyl-substituted monocarboxylic acid is preferablyselected from saturated or unsaturated, linear or branched aliphaticcarboxylic acids which preferably have from 1 to 18 carbon atoms, morepreferably 2 to 8 carbon atoms, in particular 2 to 4 carbon atoms. Theacid may be substituted by one, two or more hydroxy groups. Examples ofthis monocarboxylic acid include formic acid, acetic acid or propionicacid or lactic acid.

The optionally hydroxyl-substituted polycarboxylicacid (e.g. diacid ortriacid) may also be substituted by one, two or more hydroxy groups. Theorganic (poly)acid may be an unsaturated (e.g. mono- or diunsaturated)or saturated, linear or branched aliphatic carboxylic acid preferablyhaving from 2 to 18 carbon atoms, more preferably 3 to 8 carbon atoms,e.g. 4 to 6 carbons atoms. Examples thereof include oxalic acid, malicacid, maleic acid, malonic acid, succinic acid, tartaric acid, citricacid or sorbic acid.

The use of optionally hydroxyl-substituted polyacids, their salts andmixtures thereof is preferred. These polyacids are preferably employedin a partially neutralized state and thus capable to act as buffer. Thedegree of neutralization preferably ranges from 15 to 95% of thecarboxyl groups and is more preferably 30 to 90%, e.g. 50 to 80%. Suchpartially neutralized polycarboxylic acids can also be provided bymixing polyacid and the corresponding salt in the necessary molar ratio.

Generally it is preferred to select among the above mono- and polyacidsweak acids, in particular those having a pK value of at least 1.5, morepreferably at least 2, even more preferably at least 3, e.g. 4 to 5 (forpolyacids the pK1 value) measured in water at 25° C.

Most preferably, the acidifying agent is selected from aqueous solutionsof citric acid, oxalic acid, lactic acid, malic acid, malonic acid,maleic acid, succinic acid, tartaric acid, sorbic acid, formic acid,salts thereof, and mixtures thereof. The most preferred acidifying agentfor use in the present invention is citric acid and its salts.

The acidic CF, in particular the acidic fluff CF can be obtained bytreating standard (fluff) CF with a solution of the acidifying agent.The same is preferably used in a concentration of 0.5 to 10 weight-% andpreferably furnishes a pH of about 2 to 6, in particular 3 to 5.Desirably, the concentration of acidifying agent is selected such thatthe weight ratio of acidifying agent(s) to dry CF is about 1 to 20%, inparticular 3 to 10%. The solution used for the treatment is preferablyaqueous although volatile organic solvents may also be used as thisfacilitates the drying of the (fluff) CF.

The treatment of the standard (fluff) CF with the solution of theacidifying agent is achieved by combining standard (fluff) CF with thesolution of the acidifying agent (e.g. by preparing a slurry, dipping orspraying) followed by the preferred steps of mixing and/or drying themixture, followed by an optional fiberization step to break apartpossibly aggregated fibers. Said drying may be achieved by letting thetreated fibers stand at ambient air or preferably by heating, forinstance to 50 to 95° C. Suitable heating conditions are also disclosedin U.S. Pat. No. 6,852,904 (col. 5, lines 30 to 53). The treatment ispreferably done by the pulp manufacturer since this obviates theadditional step of treating standard fluff pulp by the manufacturer ofthe absorbent article.

As to suitable acidic (fluff) cellulosic fibers, reference can also bemade to U.S. Pat. No. 6,852,904 B2.

Very low amounts of organic zinc salts cooperate already with acidic(fluff) CF in a very efficient odour control. A preferred lower weightlimit of organic zinc salt (calculated as zinc) seems to be at least10⁻⁵ g per g dry (fluff) CF. Herein the term “dry” used in relation toacidic (fluff) CF is to be understood such that no water has been addedto the acidic SAP and that the only water present in the acidic (fluff)CF is the unavoidable residual water from manufacturing. For the purposeof the present application, an acidic (fluff) CF or an absorbent core ispreferably regarded as “dry” after a circular test sample thereof havinga thickness of 5 to 6 mm, a diameter of 5 cm and which has beencompressed to a bulk of about 8-10 cm³/g has been kept for at least oneweek at ambient temperature (e.g. 20° C.) and a specific relativehumidity, e.g. 50% RH.

More preferably, the organic zinc salt is present in amounts of at least5×10⁻⁵ g, even more preferably at least 10⁻⁴ g, even more preferably atleast 5×10⁻⁴ g, even more preferably at least 10⁻³ g per g acidic(fluff) CF. There is no specific upper limit, even though for economicreasons, a point may be reached where it may no longer be useful tofurther increase the zinc content, for instance beyond values of 0.1 or1 g zinc per g acidic (fluff) CF, if this is not accompanied by anenhanced odour suppression.

The amount of organic zinc salt in the absorbent core is also notspecifically limited. However, the amount is preferably at least 1×10⁻⁵,more preferably at least 1×10⁻⁴, most preferably at least 5×10⁻⁴ g Znper g dry absorbent core.

There are also no specific restrictions regarding the organic zinc saltto be used. In accordance with one embodiment of the present invention,at least one zinc salt of an organic carboxylic acid, in particularmonocarboxyllic acid, having preferably 2 to 30 carbon atoms, inparticular 12 to 24 carbon atoms is used. The carboxylic acid group maybe attached to aliphatic, aliphatic-aromatic, aromatic-aliphatic,alicyclic, or aromatic residues, wherein the aliphatic chain or thealicyclic ring(s) may be unsaturated and are optionally substituted, forinstance by hydroxy or C1 to C4 alkyl. These salts include zinc acetate,zinc lactate, zinc ricinoleate and zinc abietate. More preferably, thezinc salt is the zinc salt of an unsaturated hydroxylated fatty acidhaving 8 to 18 carbon atoms. Although there is no specific restrictionregarding the number of unsaturated double bonds or hydroxy groups,those fatty acids having one or two unsaturated double bonds and one ortwo hydroxyl groups seem to be preferred. The most preferred embodimentis zinc ricinoleate. According to one embodiment of the presentinvention, the organic zinc salt is activated by means of an amino acidas in TEGO® Sorb available from Degussa.

The organic zinc salt to be used in the present invention may also becapable of removing malodorous substances chemically based on amines,e.g., nicotine in cigarette smoke, thiocompounds, e.g., allicin ingarlic and onions, and acids, e.g., isovaleric acid in human sweat, andbutyric acid. For instance, zinc ricinoleate which is, e.g., marketed byDegussa under the tradename TEGO® Sorb has the described additional odorremoving effect apart from removing ammonia.

The present invention is also not subject to any limitations regardingthe technique of incorporating the organic zinc salt into the absorbentcore. Dipping and spraying are preferred.

For instance, it is conceivable to treat the fibers [acidic (fluff) CF,optionally in admixture with non-acidic (fluff) CF] present in theabsorbent core with a solution of the organic zinc salt prior to, duringor after admixture with other absorbent materials such as SAP and priorto, during or after formation of the absorbent core from said absorbentmaterials.

-   -   According to one preferred embodiment, acidic (fluff) cellulosic        fibers, optionally in admixture with non-acidic (fluff) CF are        treated as such, i.e. in the absence of other absorbent        materials, with a solution of the organic zinc salt.    -   Alternatively, standard (fluff) CF are treated simultaneously        (e.g. by spraying, preparing a slurry, or dipping) with        acidifying agent and organic zinc salt. Then, the        above-mentioned, preferably aqueous solution containing the        acidifying agent also includes the organic zinc salt, in        particular the zinc salt of a monocarboxylic acid such as zinc        ricinoleate as second component. The zinc salt is preferably        contained in amounts leading to the above disclosed Zn contents        per dry acidic CF. Regarding other treatment conditions,        reference can be made to the above description of manufacturing        acidic (fluff) CF.

Both techniques can be equally carried out with the (fluff) CF fibers(for instance by preparing a slurry, spraying or dipping the fibers intosaid solution) and (fluff) CF sheets (e.g. by dipping or spraying) asprepared by the manufacturer prior to the delivery of the sheets to themanufacturer of the absorbent articles. These two techniques areespecially preferred since they avoid the extra step of spraying theorganic zinc salt solution when manufacturing the absorbent article. Theother optionally present absorbent materials such as SAP are then addedduring or after formation of the absorbent core.

Preferably, the cellulosic fibers and/or the SAP are pretreated byadding a solution of the acidifying agent and organic zinc salt, andthen these are incorporated into the absorbent core during coreformation.

According to the above spraying techniques, the solution containing theorganic zinc salt, in particular zinc ricinoleate can be sprayed on oneor both sides of the absorbent core, or one of both sides of individuallayers constituting the same.

The solvent used for the solution of organic zinc salt can be water, apreferably volatile organic solvent such as ethanol or a mixture ofwater and a water-miscible organic solvent such as ethanol. Preferably,the organic zinc solvent is present in the solution in a relatively highconcentration, preferably 1 to 30 wt.-%. The use of such concentratedsolutions ensures that the absorption capacity of the superabsorbentmaterial is not impaired more than necessary. Commercially availablesolutions of organic zinc salts such as TEGO® Sorb A30 available fromDegussa (content of actives 30 weight %, zinc ricinoleate activated byan amino acid) can also be employed.

The backsheet typically prevents the exudates absorbed by the absorbentlayer and contained within the article from soiling other externalarticles that may contact the absorbent article, such as bed sheets andundergarments. In preferred embodiments, the backsheet is substantiallyimpervious to liquids (e.g., urine) and comprises a laminate of anonwoven and a thin plastic film such as a thermoplastic film having athickness of about 0.012 mm to about 0.051 mm. Suitable backsheet filmsinclude those manufactured by Tredegar Industries Inc. of Terre Haute,Ind. and sold under the trade names X15306, X10962, and X10964. Othersuitable backsheet materials may include breathable materials thatpermit vapors to escape from the absorbent article while stillpreventing exudates from passing through the backsheet. Exemplarybreathable materials may include materials such as woven webs, nonwovenwebs, composite materials such as film-coated nonwoven webs, andmicroporous films. Since there is always a trade-off betweenbreathability and liquid-impermeability it can be desired to providebacksheets showing a certain, relatively minor liquid-permeability butvery high breathability values.

The above elements of an absorbent article can be assembled, optionallytogether with other typical elements of absorbent articles in a mannerknown in the art.

The present invention also relates to acidic cellulosic fibers having apH of 5.5 or less characterized in that they comprise the zinc salt of amonocarboxylic acid. In this organic zinc salt the monocarboxylic acidpreferably has the features stated above. Most preferably the zinc saltis zinc ricinoleate.

Similarly the above description of cellulosic fibers, in particularfluff pulp fibers and techniques for acidification is fully applicableto the claimed acidic cellulosic fibers. According to one embodiment,these are obtainable by treating cellulosic fibers with an acidifyingagent (as described above) and the zinc salt of a monocarboxylic acid.According to another embodiment, the acidifying agent is present in anamount of 1 to 20 weight-% based on the dry weight of the untreatedfibers.

The present invention also extends to the use of such acidic cellulosicfibers for odour control, preferably in those areas where bacterialcontrol is an issue, including absorbent articles as claimed as wellwipes, such as wipes for the feminine hygiene, baby wipes, medical wipesand wipes for cleaning bathroom equipment, e.g. toilets; bandages;underpads; absorbent drapes; underpants etc. Their use for odour-controlin absorbent articles of the above-described type is preferred.

The following examples and comparative examples illustrate the presentinvention.

Examples

Test Methods

A) pH of Absorbent Core

The pH of the absorbent core can be measured very precisely with thefollowing method involving the preparation of a test absorbent core andpH measurement using the same.

Method 1: Preparation of Absorbent Cores for Test

Absorbent cores were punched out of an absorbent core produced in apilot plant. A standard method of mat forming a core was used in theproduction of the core in the pilot plant. The absorbent core consistedof a homogenous mixture of acidic (fluffed) pulp and optionallysuperabsorbent material. The absorbent core was compressed to a bulk ofabout 8-10 cm³/g. The size of the punched cores was 5 cm in diameter,the weight of the same about 1.2 g.

Method 2: Measurement of pH in an Absorbent Core

An absorbent core having a diameter of approximately 50 mm was preparedaccording to Method 1. A predetermined amount of Test liquid 1 wasadded, 16 ml to all samples, whereafter the absorbent core was left toswell for 30 minutes. Thereafter, pH was measured on the liquid squeezedout of the samples using a surface electrode, Flat-bottomed, type SinglePore Flat, Hamilton. The results of three tests were averaged for themeasurement.

Test Liquid 1 (Referred to in Method 2):

Synthetic urine containing the following substances: KCl, NaCl, MgSO₄,KH₂PO₄, Na₂HPO₄, NH₂CONH₂. The pH in this composition is 6.0±0.5.

The test liquid to be used is 16 ml synthetic urine (as defined above)for each core absorbent body.

Example 1

Circular test absorbent cores having a weight of about 1.16 g and adiameter of 5 cm were punched out of an absorbent core produced in apilot plant. A standard method of mat forming a core was used in theproduction of the core in the pilot plant. The absorbent core consistedof a homogenous mixture of acidic fluff pulp and superabsorbentmaterial. The fluff pulp used was 0.69 g Weyerhaeuser acidic fluff pulpand the superabsorbent material was 0.47 g of a superabsorber (SXM 9155,Degussa). The acidic fluff pulp is commercially available fromWeyerhaeuser under the material description TR118 and manufactured bytreating ECF Kraft pulp based on 100% US Southern pine wood with 4%citric acid and 1% citrate as an additive. It has a pH of 3.4±0.2. ThepH of the acidic fluff pulp was measured in accordance with the standardTappi T 509-02. More specifically, the above pH value is the 5 minutespulp sheet pH based on Tappi method T 509 om-02. The absorbent core wascompressed to a bulk of about 8-10 cm³/g.

To the absorbent core 1.3 ml of a 0.5 wt.-% solution of zinc ricinoleate(available from Degussa under the tradename TEGO® Sorb A30, suitablydiluted) was added by either dripping the solution onto the surface (onone side) or dipping one side of the core into the solution. The treatedabsorbent body was left standing at ambient air for one week. Thisprocedure led to a concentration of 5.55×10⁻⁴ g Zn per g dry absorbentcore. Then, the absorbent body was allowed to absorb 16 ml syntheticurine according to Method 3 as described below and allowed to stand atroom temperature.

6 h and 8 h after the absorption of synthetic urine the amount ofammonia developed was measured.

Five measurements were averaged as mean value. The results are shown inTable 1.

Method 3: Measurement of Ammonia Inhibition in Absorbent Cores

Absorbent cores were prepared in accordance with Method 1. Test liquid 2was prepared. Bacteria suspension of Proteus mirabilis was cultivated innutrient broth 30° C. overnight. The graft cultures were diluted and thebacterial count was determined. The final culture containedapproximately 10⁵ organisms per ml of test liquid. The absorbent corewas placed in a plastic jar and the Test liquid 2 was added to theabsorbent core, whereafter the container was incubated at 35° C. 6 and 8hours respectively, whereafter samples were taken from the containersusing a hand pump and a so called Dräger-tube. The ammonia content wasobtained as a colour change on a scale graded in ppm or volume percent.

Test Liquid 2:

Sterile synthetic urine to which has been added a growth medium formicro-organisms. The synthetic urine contains mono- and divalent cationsand anions and urea and has been prepared in accordance with theinformation in Geigy, Scientific Tables, Vol 2, 8^(th) ed. 1981 p. 53.The growth medium for the micro-organisms is based on information ofHook- and FSA-media for entero-bacteria. The pH in this mixture is 6.6.

Comparative Example 1

An absorbent core was formed in the same manner as in Example 1, withthe sole exception that a treatment with a solution of zinc ricinoleatewas not carried out.

Comparative Example 2

An absorbent body was formed in the same manner as in Example 1 with thedifference that a 6 wt.-% solution of zinc ricinoleate was used and theacidic fluff pulp was replaced with a standard fluff pulp (NB 416 fromWeyerhaeuser). This procedure led to an amount of 6.66×10⁻³ g Zn per gdry absorbent core.

The results in terms of ammonia formation of Example 1 and ComparativeExamples 1 and 2 are shown in the following Table 1.

TABLE 1 ammonia ammonia formation formation sample description (ppm) 6 h(ppm) 8 h CEx 1 Acidic fluff pulp¹ 38 760 CEx 2 Zn² + non-acidic fluff<19 270 pulp³ Ex 1 Acidic fluff pulp¹ + Zn² 1 16 ¹Acidic fluff pulp(Weyerhaeuser, pH 3.4) ²Zinc ricinoleate ³NB 416 (Weyerhaeuser)

The above experiments show that the combined use of an acidic fluff pulpand an organic zinc salt such as zinc ricinoleate suppresses theformation of ammonia to a very surprising extent. Considering the factthat a human can vaguely detect the smell of ammonium at a concentrationof 150 ppm, the present invention ensures that during use of anabsorbent article, no ammonia odour will be perceived by the wearer.

1. Absorbent article comprising a liquid-permeable topsheet, a backsheetand an absorbent core enclosed between said liquid-permeable topsheetand said backsheet, wherein said absorbent core comprises acidiccellulosic fibers having a pH value of 5.5 or less, and an organic zincsalt.
 2. Absorbent article according to claim 1, wherein said cellulosicfibers are fluff pulp fibers.
 3. Absorbent article according to claim 1,wherein the acidic cellulosic fibers comprised in the absorbent core areobtained by acidifying cellulosic fibers with an acidifying agent. 4.Absorbent article according to claim 3, wherein the acidifying agent isan organic acid having a pK value of at least 1.5 (measured in water at25° C.).
 5. Absorbent article according to claim 3, wherein theacidifying agent is selected from aqueous solutions of citric acid,oxalic acid, lactic acid, malic acid, malonic acid, maleic acid,succinic acid, tartaric acid, sorbic acid, formic acid, salts thereof,and mixtures thereof.
 6. Absorbent article according to claim 1, whereinthe acidic fluff pulp has a pH value of 5.0 or less.
 7. Absorbentarticle according to claim 1, wherein the acidic fluff pulp has a pHvalue of 2.0 to 5.0.
 8. Absorbent article according to claim 1, whereinthe amount of organic zinc salt is at least 10⁻⁵ g Zn per g dry acidiccellulosic fibers.
 9. Absorbent article according to claim 1, obtainedby treating the absorbent core or the acidic cellulosic fibers containedtherein with a solution of the organic zinc salt.
 10. Absorbent articleaccording to claim 1, wherein the organic zinc salt is selected fromzinc salts of carboxylic acids having 2 to 30 carbon atoms. 11.Absorbent article according to claim 10, wherein the carboxylic acidrepresents an unsaturated hydroxylated fatty acid having 8 to 18 carbonatoms.
 12. Absorbent article according to claim 10, wherein the zincsalt is zinc ricinoleate.
 13. Absorbent article according to claim 1,wherein the backsheet is liquid-impermeable.
 14. Acidic cellulosicfibers having a pH of 5.5 or less, comprising a zinc salt of amonocarboxylic acid.
 15. Acidic cellulosic fibers according to claim 14,wherein said monocarboxylic acid represents an unsaturated hydroxylatedfatty acid having 8 to 18 carbon atoms.
 16. Acidic cellulosic fibersaccording to claim 14, wherein said organic zinc salt is zincricinoleate.
 17. Acidic cellulosic fibers according to claim 14,obtained by treating cellulosic fibers with an acidifying agent and thezinc salt of a monocarboxylic acid.
 18. Acidic cellulosic fibersaccording to claim 14, wherein said acidifying agent is selected amongorganic acids having a pK value (water, 25° C.) of at least 1.5. 19.Acidic cellulosic fibers according to claim 18, wherein said acidifyingagent is present in an amount of 1 to 20 wt. % based on the dry weightof the untreated fibers.
 20. Acidic cellulosic fibers according to claim14, wherein the cellulosic fibers are fluff pulp fibers.
 21. A methodfor controlling odour comprising utilizing the acidic cellulosic fibersaccording to claim
 14. 22. A method according to claim 21 comprisingcontrolling odour in absorbent articles.
 23. Absorbent article accordingto claim 1, wherein the absorbent article is a diaper, panty diaper,sanitary napkin or incontinence device.
 24. Absorbent article accordingto claim 1, wherein the acidic fluff pulp has a pH value of 3.0 to 4.0.